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Revolving Line of Credit (Detail) - USD ($)
6 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Nov. 16, 2015
Line Of Credit Facility [Line Items]      
Effective percentage interest rate 3.65%    
Outstanding borrowings $ 15,000,000 $ 0  
Revolving Credit Facility | Credit Agreement      
Line Of Credit Facility [Line Items]      
Revolving credit facility, maximum borrowing capacity     $ 300,000,000
Outstanding borrowings 15,000,000.0    
Line of credit facility, unused borrowing capacity $ 285,000,000.0    
Revolving Credit Facility | Credit Agreement | Prime Commercial Lending Rate      
Line Of Credit Facility [Line Items]      
Effective percentage interest rate 5.75%    
Revolving Credit Facility | Credit Agreement | Minimum | Prime Commercial Lending Rate      
Line Of Credit Facility [Line Items]      
Line of credit facility interest rate 0.25%    
Revolving Credit Facility | Credit Agreement | Minimum | Libor Rate      
Line Of Credit Facility [Line Items]      
Line of credit facility interest rate 1.25%    
Revolving Credit Facility | Credit Agreement | Maximum | Prime Commercial Lending Rate      
Line Of Credit Facility [Line Items]      
Line of credit facility interest rate 1.00%    
Revolving Credit Facility | Credit Agreement | Maximum | Libor Rate      
Line Of Credit Facility [Line Items]      
Line of credit facility interest rate 2.00%    
Revolving Credit Facility | Credit Agreement | Letter of Credit      
Line Of Credit Facility [Line Items]      
Revolving credit facility, maximum borrowing capacity     $ 10,000,000